1. Field of the Invention
The invention relates to a method for transcoding a data stream and to a corresponding transcoding device.
2. Description of the Related Art
Video coding methods enable large data sets of image material to be compressed with different degrees of quality and at different data rates. It is often necessary in this case to adapt the data in a modified form to a transmission or to a terminal device. In particular it may be necessary to convert the data stream of one video coding standard into a data stream of another video coding standard. A conversion of the data stream of this kind is referred to as transcoding.
Transcoding methods in which a data stream is fully decoded and subsequently encoded in a new coding standard are known from the prior art. This approach leads to losses in terms of quality and is also very complex as a result of the decoding and subsequent recoding, resulting in a negative effect on the duration of the conversion of the data stream.
A reduction in the complexity of currently popular transcoding methods is achieved in P. N. Tudor and O. H. Werner; Real-Time Transcoding of MPEG-2 Video Bit Streams; International Broadcasting Convention; Amsterdam; September 1997; pp. 286-301, and O. H. Werner; Generic Quantiser for Transcoding of Hybrid Video; Proceedings of the Picture Coding Symposium; Berlin; September 1997, by reducing the computational overhead for the motion estimation. With the approaches proposed in these documents, the prediction vectors normally used for the recoding, by which the movement in the current image is predicted from an image preceding it in time, is re-estimated only in a greatly reduced search area rather than in the entire image. The reduced search area comprises only a few pixels or only half-pixel or quarter-pixel environments. It has been shown that as a result, the degree of complexity can be considerably reduced and at the same time the quality of the transcoded data stream is made only slightly worse.
The above-described improvement in transcoding methods is suitable in particular for transcodings which take place within the same compression standard. A method is also known from N. Björk, Ch. Christopoulos; Video Transcoding for Universal Multimedia Access; Proceedings of the ACM Multimedia 2000; Marina del Rey; October-November 2000, wherein a transcoding is made possible in the same compression standard, with the motion vectors used additionally being scaled in accordance with a change in the image size and subsequently being re-estimated in turn in a reduced search area. Further approaches for transcoding from one standard to another standard are known from N. Feamster and S. Wee; An MPEG-2 to H.263 Transcoder; Proceedings of Symposium and Voice, Video, and Data Communications; Boston; September 1999, and J. Xin et al.; Motion Reestimation for MPEG-2 to MPEG-4 Simple Profile Transcoding Proceedings of the International Packet Video Workshop; 2002.
The standard methods according to the prior art simplify the transcoding of the data stream for coding methods in which a temporal prediction of the image blocks of a digitized image takes place. However, new compressions methods increasingly use what are referred to as intra-prediction methods in which the individual image blocks within a digitized image are predicted locally from already coded image blocks in the same image. With an intra-prediction of this kind compression is improved further.
To date, no methods from the prior art are known by which an economical transcoding entailing little effort is ensured for a compression standard using intra-prediction.